The present disclosure relates to a technology of a developing roller that is disposed to face, without contact, the outer circumferential surface of the image carrier.
A developing device is installed in an image forming apparatus which is a copier, a printer or the like and forms an image on a paper sheet based on the electrophotography. The developing device develops, by toner, an electrostatic latent image formed on an image carrier such as a photoconductor drum. As the developing method, a so-called two-component developing method is known which uses two-component developer including magnetic carrier and toner to develop a toner image on the image carrier. As an example of the two-component developing method, there is conventionally known a contactless developing system called “interactive touchdown developing system”. In the interactive touchdown developing system, a developing roller and a magnetic roller are used. The developing roller is disposed at a predetermined distance from the image carrier. A magnet is embedded in the magnetic roller. The magnetic roller draws up the magnetic carrier as well as the toner, and holds them on the surface thereof. The magnetic roller forms a magnetic brush thereon to transfer only the toner to the developing roller, and form a toner thin layer on the developing roller. An AC electric field is generated by a developing bias including an AC component applied to the developing roller, and the AC electric field flies the toner from the developing roller and causes the toner to adhere to the electrostatic latent image on the image carrier.
Meanwhile, the developing device is required to have high developability. That is, the developing device is required to surely cause toner to be flown from the developing roller and adhered to the electrostatic latent image on the image carrier. To improve the developability, the resistance of the developing roller may be decreased to increase the magnitude of the developing electric field (an electric field between the developing roller and the photoconductor drum). However, when the resistance of the developing roller is decreased, the adhesion (image force) of toner not having been used in the developing to the surface of the developing roller increases, thereby it becomes difficult for the not-used toner to separate from the developing roller. When the toner carried on the surface of the developing roller is held there for a long time without being separated, a toner filming occurs to the surface of the developing roller, resulting in a roller contamination. On the other hand, when the image force is reduced, it becomes easy for the toner to separate, the roller contamination by the toner filming is restricted, and the contamination resistance is improved. However, when the image force is reduced, the toner easily separates from the developing roller and unnecessarily adheres to the image carrier. In that case, a toner fogging occurs and the developability by the toner is decreased.